This invention relates to the field of electronic test and measurement and more particularly to a bandpass sampling technique for analyzing wideband electrical signal spectra.
Prior signal processing and analysis systems, such as spectrum analyzers, typically employ multiple frequency conversion stages to convert a signal band of interest to a baseband signal that is low-pass filtered before being digitized by an analog-to-digital converter (xe2x80x9cADCxe2x80x9d) for further processing and display. This architectural approach places the signal frequency being measured within the first Nyquist band of the ADC to prevent aliasing of out-of-band signals into the analysis band of frequencies.
For example, U.S. Pat. No. 4,257,104 for APPARATUS FOR SPECTRUM ANALYSIS OF AN ELECTRICAL SIGNAL describes a fully featured spectrum analyzer system that employs multiple down converters, intermediate frequency (xe2x80x9cIFxe2x80x9d) amplifiers, a detector, and video processor prior to the ADC. Simpler architectural approaches are described in U.S. Pat. Nos. 4,607,215 for SPECTRUM ANALYZER, 4,890,099 for SIGNAL ANALYZER HAVING PARTIALLY SCROLLING FUNCTION FOR DISPLAY SCREEN, 4,839,582 for SIGNAL ANALYZER APPARATUS WITH AUTOMATIC FREQUENCY MEASURING FUNCTION, and 5,038,096 for SPECTRUM ANALYZER CIRCUIT FOR PULSED INPUT SIGNALS. However, all of these architectural approaches describe signal down conversion, IF amplification, filtering, and detection of a baseband of frequencies prior to digitization for subsequent signal processing and analysis.
In another example, U.S. Pat. No. 5,629,703 for METHOD FOR REDUCING HARMONIC DISTORTION IN AN ANALOG-TO-DIGITAL CONVERTER SYSTEM, which is assigned to the assignee of this application, describes a spectrum analysis architecture employing down conversion to a baseband of frequencies and low-pass filtering, but does not employ detection prior to digitization. The low-pass filtering limits the signal analysis bandwidth to a relatively narrow 9 MHz.
The above-described signal measurement systems are not only complex and costly, but also require a wide local oscillator (xe2x80x9cLOxe2x80x9d) tuning or sweeping range to achieve a usefully wide signal analysis bandwidth. Such signal measurement systems further require an ADC having a relatively high sampling rate to achieve alias-free signal digitizing within the first Nyquist band.
What is needed, therefore, is an electrical signal analysis architecture having reduced complexity and cost while achieving a wide signal analysis bandwidth with a relatively narrow LO tuning range and a relatively low ADC sampling rate.
An object of this invention is, therefore, to provide an apparatus and a method for analyzing electrical signals.
Another object of this invention is to provide a simpler and less costly spectrum analyzer architecture than is found in prior architectures.
A further object of this invention is to provide a spectrum analyzer having a wide signal analysis bandwidth while requiring a relatively narrow LO tuning range and a relatively low ADC sampling rate.
A bandpass sampling signal processing and analysis architecture of this invention employs a conventional mixer driven by a frequency-tunable LO to upconvert a signal frequency band to an IF frequency band that is above the signal frequency band. The IF frequency band is passed through an IF bandpass filter to provide to subsequent digitization stages an IF bandpass range of signal frequency components. In this architecture, the IF bandpass filter acts as an anti-alias filter for the digitization stages. The LO frequency is selected to place the upconverted signal frequency band within the pass band of the IF bandpass filter. The resultant IF bandpass signal is sampled and digitized at a rate that is commensurate with the IF bandwidth, but typically much lower than the IF bandpass center frequency. Signal sampling is carried out by a sample-and-hold or a track-and-hold circuit, the output of which is applied to an ADC that is clocked at the same rate as the sampling circuit.
The bandpass sampling architecture of this invention is advantageous because it is inherently less complex and costly than prior down conversion and base band sampling architectures.
The bandpass sampling architecture of this invention is also advantageous because upconversion of the signal frequency band provides a wide signal analysis bandwidth while requiring only a relatively narrow (percentage-wise) LO tuning range (typically less than an octave).
The bandpass sampling architecture of this invention is further advantageous because sampling of the IF bandpass signal prior to digitizing allows using an ADC having a much lower sampling rate than is ordinarily required for alias-free sampling within the first Nyquist band.
Additional objects and advantages of this invention will be apparent from the following detailed description of a preferred embodiment thereof that proceeds with reference to the accompanying drawings.